Back spot facing tools of the above mentioned kind are previously known through U.S. Pat. No. 3,540,324. These tools have proved to be exceptionally good concerning both function, safety of operation and applicability. In some cases difficulties can appear when the wing during rotation is to be retracted to inactive position in order to introduce the spindle through a boring in a workpiece, the rear side of which is to be provided with a spot recess or the like. Irrespective of the rotational direction the wing will through the centrifugal force be pivoted out to an active position, and if the wing is to be retracted into a recess int he spindle, the front end surface of the wing is brought to contact with the workpiece at which normally a retraction of the wing to inactive position takes place. If the friction between the wing and the workpiece is not sufficiently high, for example if there is oil on the workpiece and/or the workpiece is smooth after the boring operation, the wing will remain in its extended position even at high pressures against the workpiece and the spindle can therefore not be passed into the boring. The retraction of the wing has up to now occured because of the friction between the front end surface of the wing and the contact surface on the workpiece outside the hole.